The present invention relates to a method and system for purifying exhaust gas from an engine, and more particularly to a method and system for purifying exhaust gas by supplying an additive agent as a reducing agent to a selective reducing catalyst provided in an exhaust passage from an engine.
Conventionally, an exhaust gas purification system has been known which includes a selective reducing catalyst in an exhaust passage from a diesel engine to purify exhaust gas from the engine, as disclosed in Japanese Patent Application Publication 2009-167940. The selective reducing catalyst induces the selective reaction between NOx and a reducing agent even in a coexistent condition with oxygen. The system ejects aqueous urea solution as the reducing agent from the upstream side of the selective reducing catalyst to supply the ejected aqueous urea solution to the catalyst. The ejected aqueous urea solution undergoes pyrolysis under the heat of the exhaust gas or hydrolysis, forming ammonia and carbonic acid gas. The ammonia serves to reduce NOx in the exhaust gas on the selective reducing catalyst.
The system disclosed in the publication additionally includes a diesel particulate filter (hereinafter referred to as DPF) to trap particles in the exhaust gas at the upstream side of the selective reducing catalyst and an oxidation catalyst at the upstream side of the DPF. The oxidation catalyst oxidizes the NO in the exhaust gas into NO2. Then, in the DPF, the carbon components of the particles and the NO2 (including both NO2 originally contained in the exhaust gas and NO2 formed by the function of the oxidation catalyst) react with each other, forming NO and CO2.
In the meantime, the oxidation catalyst provided at the upstream side of the DPF degrades through prolonged exposure to high temperature and its oxidation capability for NO drops correspondingly to the time period. The drop in oxidation capability of the catalyst leads to a decrease in the amount of NO2. As the amount of NO2 formed by the function of the oxidation catalyst decreases, a concentration ratio of NO to NO2 in the exhaust gas at the downstream side of the DPF increases. For the selective reducing catalyst, on the other hand, the amount of ammonia necessary to decompose NO is smaller than that necessary to decompose NO2. If the supply amount of aqueous urea solution (i.e., ammonia) is set with no consideration for the decrease in the amount of NO2 formed by the function of the oxidation catalyst, accordingly, the amount of ammonia unnecessary for NOx purification gradually increases. As a result, the unwanted surplus ammonia may be adversely released into the atmosphere.
In view of the abovementioned problem, it is an object of the present invention to decrease the amount of surplus additive agent unnecessary for NOx purification by supplying a proper amount of the additive agent to a selective reducing catalyst even when an oxidation catalyst at the upstream side of the selective reducing catalyst has degraded.